1. Field of the Invention
The present invention relates to phase detectors for high power distribution networks, and more particularly to remote devices which may be used to identify which phase in a three-phase power system is carried by a line.
2. Description of the Related Art
As is well known in the art, conventional high-power distribution networks carry voltages in three phases, generally identified as the A, B and C phases. The three voltage phases typically consist of 50 Hz or 60 Hz sine waves which are out of phase by approximately 120.degree.. The three phases are generated at a power plant or other power source and distributed across a wide service area to provide power to multiple residences and businesses. A three-phase power transmission line generally has three transmission conductors, one for each phase, and sometimes a fourth conductor which is neutral.
Many utility companies experience difficulties which arise from the inability of field personnel to accurately identify the phase of a transmission line on an unmarked cable in an underground vault in an underground system. The inability to identify a given conductor causes operational problems. Specifically, the lack of proper identification of the voltage phase carried by remote voltage conductors causes difficulty when customer loads have to be shifted from one phase to another under emergency conditions, or when a planner is preparing for future load expansion. Both situations require proper load balancing among the three phases along the distribution network to operate the network effectively. In short, reliable phase identification would allow field personnel to perform their functions with increased confidence and effectiveness.
Devices developed to identify the phases of signals carried by unmarked power lines have been cumbersome and often unacceptable with regard to the accuracy of the measurements provided by these devices. For example, one device, disclosed in U.S. Pat. No. 4,626,622, proposes the identification of an unknown phase via a telephone or cellular phone connector to a circuit having a known phase. The known phase of one of the voltage signals is digitized and transmitted over the telephone system. The data is received by a remote unit and reconverted into a reference signal which is representative of the transmitted voltage signal having a known phase. The reference signal is compared with the signal having an unknown phase to determine the phase of the unknown signal.
A number of complications are associated with such systems. Specifically, because a series of different data conversions and transmissions is required to provide a reference signal, and because these conversions and transmissions involve inductive and/or capacitive delays, a significant phase difference exists between the reference signal and the actual known signal from which the reference signal was derived. In the system described in U.S. Pat. No. 4,626,622, a calibration is performed at each remote location to compensate for the phase lag introduced at various remote locations. However, because telephone companies sometimes operate by route switching to increase efficiency, signals on telephone lines are sometimes rerouted over circuits which may vary by hundreds of miles. The additional phase shift introduced by the time delay associated with such rerouting may change after calibration. Thus, present phase identification systems can be unreliable due to the necessity of calibration at each remote location and due to the possible change in delays introduced by telephone line rerouting.
In addition, phase identification systems often digitize the known reference signal. In order to preserve an accurate representation of the known analog sine wave, a high bit resolution, and thereby a high transmission bandwidth, is required. However, a high transmission bandwidth is often not provided by ordinary telephone connections which typically have a bandwidth of 3 KhZ or less. Consequently, any telephone transmission of a digitized reference signal is typically low resolution, or only uses a single "ping" transmitted at every zero crossing. Thus, the reference signal is not transmitted in its entirety. The limited bandwidth compromises the flexibility of such systems.